In this study we have identified experimental conditions that will support the assessment of tear microdesiccates by variants of light microscopy. To date, studies involving characterization of single tear microdesiccates have frequently used either darkfield microscopy or phase contrast microscopy and have been exclusively focused on assessing either presence or absence of fern-like crystalloids. Furthermore, consideration of any other structural element of microdesiccates being formed during tear water evaporation has been disregarded consistently [1, 8–11]. Recent studies using light microscopy have shown that a normal tear microdesiccate comprises several annularly distributed morphological domains or zones, namely zones II and III (two highly distinctive zones whose organization is based on fern-like crystalloids), a transition band (a narrow compact band whose structure is based on rope-like elements) and zone I (a translucent and barely visible outer circle of desiccates) . By making adjustments in the method of focusing light onto the dry tear specimen by means of an Abbé condenser with 5-position turret (brightfield, phase contrasts 1, 2 and 3 and darkfield) we have now been successful in identifying optimal conditions for the differential (and simultaneous) observation of some of those structural domains of tear microdesiccates. Thus, by using illumination systems other than those of the usual darkfield or phase contrast microscopy, zone I became clearly noticeable as an architectural distinct component of normal tear microdesiccates. To date, zone I of tear microdesiccates has gone mostly unnoticed among tear fern analysts despite it was originally described in 1955 by Solé as an amorphous and barely visible structure which can be penetrated by tiny rod-like elements [14–16]. Furthermore, by means of finer adjustments consisting in positioning the condenser in intermediate positions between brightfield and phase 1, or between phases 1 and 2, both fern-like crystalloids and zone I, that is, the two most distinctive elements of a normal tear microdesiccate, could be seen simultaneously and with a properly balanced resolution.
An analysis of the specialized literature shows that practically none of the reports concerning tear microdesiccates—with the exception of that of Horwath et al. —present either whole tear microdesiccates or their outer zone I [17–21]. Moreover, descriptions in those reports are referred only to presence or absence of tear fern-like crystalloids. Such observational bias of researchers and clinicians using light microscopy of tear microdesiccates as a diagnostic test for the assessment of the ocular surface seems to derive from very different sources. Firstly, production of tear microdesiccates on glass surfaces and their analysis by light microscopy is widely known as the tear ferning test. Certainly, such nomenclature draws attention to a single goal [10, 17, 18]. On the other hand, most of reports on tear microdesiccates do coincide in documenting just a very minor area of every single microdesiccate [2, 7, 8, 20, 21]. Considering both variety and abundance of structures and domains that are usually present in normal tear microdesiccates, such selection is by far a confounding factor. Another also important factor accounting for the only partial use of the information derived from any tear microdesiccate is the lack of standardization among the observation procedures used in different studies. Thus, observations reported to have been made at magnifications of either 10× [21–23], 40× , 25 and 125× , 40–100× , 100× , or even 400× , are hardly comparable. Moreover, reports rarely indicate the particular combination of ocular and objective lenses used in the observations, which can also preclude comparisons and be of great significance for appropriate data collection. Quite often, the areas of the fields of view corresponding to the above-mentioned range of magnifications do oblige authors to select a fraction of the tear microdesiccate to be exhibited as representative of the whole specimen. A quite closely related aspect leading to the same biased result can derive from the use of relatively large tear volumes to produce microdesiccates, so that it becomes impractical or impossible to watch the whole specimen. In our experience, microdesiccates produced with tear volumes equal or higher than 2 µL can hardly be seen under a common light microscope whose lowest power objective lens is usually around 4–5× . Unfortunately, data on the volume of tear used to produce microdesiccates are rarely communicated in specialized reports. Also in reference to methodological aspects that may restrict markedly the information provided by a tear microdesiccate is the use of some particular types of light microscope techniques. Among studies dealing with characterization of tear microdesiccates, a majority involved the transmitted-light darkfield microscopy variant [12, 13, 17], while others used either phase contrast microscopy [24, 25], visible light microscopy [9, 10] or polarized-light microscopy . Some reports do not provide sufficient technical data in this respect . Morphological information obtained by using those different experimental approaches can differ markedly. Concerning tear microdesiccates we have now shown that darkfield microscopy enhances imaging of fern-like crystalloids but, in turn, makes zone I practically invisible.
In an already classical report aimed to systematize the assessment of tear microdesiccates, Rolando proposed the use of a 4-level numeric scale (I through IV) to evaluate the power of tear fluid to form fern-like crystalloids following spontaneous desiccation on a glass surface at ambient conditions [1, 10]. In addition, those authors showed that levels I and II (higher fern-forming capability) were more frequent among tear fluids collected from normal eyes whereas levels III and IV (lower fern-forming capability) were more common in tear fluids of patients with keratoconjunctivitis sicca [1, 10]. Given the remarkable diversity of procedures to produce and evaluate tear microdesiccates, it has not been surprising that the Rolando’s scale has been used or interpreted very differently by different authors (e.g. ref.  versus ref. ). Also, in a recent analytical study on typing tear microdesiccates in association with the tear ferning test, a new 5-point scale displaying improved discrimination, repeatability and reliability over the conventional Rolando’s scale was proposed in order to provide a better support to researchers and clinicians using the test . Such study was also focused only on the fern-like crystalloids with no consideration to any other structural element of tear microdesiccates . Despite these various technical, methodological and interpretive limitations, acceptable sensitivity and specificity values of the tear ferning test in screening Dry eye have been reported [20, 26, 27]. Certainly, the properties displayed by a tear microdesiccate should account at least partly for the quality of the tear fluid from which it is produced. In this context, the link made by Rolando between a morphological feature of tear microdesiccates and tear quality is highly valuable and should be given first consideration. In accordance with that premise, our study was aimed at defining basic experimental conditions allowing the observer to characterize whole tear microdesiccates being produced under standard conditions. Thus, the combined use of a tear volume of 1–1.5 µL to produce a microdesiccate and a 2.5× objective lens for its analysis represented primary conditions to recognize a whole microdesiccate. To resolve and characterize the main morphological domains of a tear microdesiccate the use of alternative illumination settings, in reference to the basic positions of a standard 5-position turret condenser, was found to be equally important. In this study, some of the domains of a tear microdesiccate could be consistently resolved by using particular types of illumination. In agreement with a number of previous reports, the major tear fern-like crystalloids can be properly resolved using darkfield microscopy or some types of phase contrast illumination (Ph3). However, under this type of illumination zone I of tear microdesiccated specimens becomes practically invisible. Contrarily, by using some phase contrast illuminations (Ph1) the borders of zone I become clearly demarcated but resolution of the centermost fern-like crystalloids is reduced significantly. On the other hand, because of the consistent lack of use of stains in the assessment of tear microdesiccates, brightfield microscopy has not been exploited yet for the assessment of tear microdesiccates. Accordingly, none of the standard positions of the 5-position condenser by itself has allowed to describe comprehensively a whole tear microdesiccate. In order to attain views of microdesiccates in which both the zone I and the domains displaying fern-like crystalloids were jointly resolved, additional illumination settings provided by intermediate positions between the five fix positions in the turret condenser were explored. Thus, illuminations of tear microdesiccates from healthy subjects provided by intermediate positions between the standard brightfield and phase 1 positions or between the standard phase 1 and phase 2 positions resulted in whole tear microdesiccates showing simultaneously both fern-like crystalloids of zones II and III, a compact and structured zone I and a complex transition band. Recently reported studies from our laboratory have shown that the main domains of tear microdesiccates have distinctive physicochemical characteristics . In that regard, using energy dispersive X-ray analysis (EDXA) coupled to scanning electron microscopy, Pearce and Tomlinson showed the presence of sulphur (together with K+ and Cl−) at the edge of the dried teardrop but not in the fern-like crystalloids . Thus, different domains of tear microdesiccates may contribute with particular structural or functional properties to the tear film covering the eye surface [30, 31]. In accordance with this postulate, both the occurrence of major crystalloids in zone III (a common feature among normal microdesiccates typed as Rolando’s scores I or II), together with a seemingly structured zone I (a novel feature shown in this study) can be viewed as structural elements of normal tear microdesiccates whose scrutiny may shed some light on tear quality. Altogether, the assessment of whole tear microdesiccates may become a highly valuable source of information on normality or abnormality of the tear fluid. Far from contradicting the Rolando’s link between an altered score in the tear ferning test and physiopathological abnormality of the tear fluid, our findings do complement, enrich and diversify the possibilities of linking advantageously the morphology of whole tear microdesiccates with structural, compositional and functional aspects of tear fluid in individual patients and eyes. Clinical research in that direction should shed important lights on this new consideration of tear microdesiccates.